From the Super Mario Wiki
Herbert is a Boo in Paper Mario. When Mario and his partners arrived in Gusty Gulch, Herbert had already been eaten by Tubba Blubba. After Mario and his friends defeated Tubba Blubba, Tubba Blubba released all the Boos he had eaten and one of the Boos he released was Herbert. The player can find him on Gusty Gulch after they defeat Tubba Blubba and he will reveal that his ancestors once lived in Tubba Blubba's Castle.